For example, as disclosed in Japanese Patent Application Laid-Open Publication No. 62-222512 (Patent Document 1), a dielectric material having a high permittivity is widely used as an electronic material of a capacitor, a semiconductor device, a light-emitting device, or the like.
A capacitor includes two electrodes, that is, an anode and a cathode. As an anode material of an electrolytic capacitor, valve metal such as aluminum and tantalum, which allows an insulating oxide layer to be generated on a surface thereof, is used. Here, the valve metal is metal covered by an oxide layer through anode oxidation, and as the valve metal, aluminum, tantalum, niobium, titanium, hafnium, zirconium, zinc, tungsten, bismuth, antimony, and the like are cited. As a cathode material, any of an electrolytic solution, an inorganic semiconductor, an organic electrically conductive substance, or a metal thin film is used. In a case where the cathode material is the electrolytic solution, it is often the case that aluminum foil whose surface area is enlarged is used as a cathode terminal.
Conventionally, some methods of increasing a capacitance of a capacitor have been proposed.
For example, as a method of enlarging surface areas of anode and cathode terminals, Japanese Patent Application Laid-Open Publication No. 2004-259932 (Patent Document 2) discloses a method in which crystalline oxide including Pb is formed so as to be evenly distributed on a surface of aluminum foil and surface dissolution caused upon etching is suppressed, thereby obtaining the enlarged surface areas after the etching.
In addition, for example, as a method of forming a film which has high adhesiveness and is thick, Japanese Patent Application Laid-Open Publication No. 2003-55796 (Patent Document 3) discloses a method in which a titanium-phosphorus based composite oxide film is attached onto a surface of an aluminum base material.
Further, for example, as a material used for configuring a capacitive element, Japanese Patent Application Laid-Open Publication No. 10-182221 (Patent Document 4) discloses an article whose temperature coefficient of a permittivity is smaller.
Furthermore, for example, as a method of increasing surface areas of foil electrodes of an electrolytic capacitor, Japanese Patent Application Laid-Open Publication No. 11-317331 (Patent Document 5) discloses a method which includes a step of placing a base material in an inert atmosphere having a pressure of approximately 10−3 Torr through approximately 10−2 Torr and a step of evaporating valve metal onto a base material under an inert atmosphere to provide the base material with a surface structure body. However, in the method in which a dielectric layer including the valve metal is formed by the evaporation, not only it is required to retain a high vacuum but also it is difficult to industrially form a dielectric layer having an even thickness.
In the meantime, by using a sol-gel method, as compared with the evaporation method, it is easy to form a dielectric layer including the valve metal and the dielectric layer having the even thickness can be industrially formed. However, in the dielectric layer obtained by using the sol-gel method, defects such as cracks are easily caused in a step of forming the dielectric layer by heating a dielectric precursor, thereby leading to, for example, the problems in that adhesiveness to aluminum as a base material is weakened, that alumina is formed between a dielectric layer and an aluminum substrate, thereby reducing a capacitance, and that a desired withstanding voltage cannot be obtained.
As a method of solving the above-mentioned problems, for example, International Publication No. WO 02/062569 (Patent Document 6) discloses a method of manufacturing a structure body having an oxide layer on a base substance. This method includes: an application liquid preparation step of preparing an application liquid for an oxide layer; a base substance surface preprocessing step of obtaining a preprocessed base substance by preprocessing a surface of a base substance, aside from the application liquid preparation step; an application step of obtaining a coated base substance by applying the application liquid for an oxide layer to the preprocessed base substance; and a firing step of forming an oxide layer on the base substance by firing the coated base substance, and the base substance surface preprocessing step includes a first process of causing a surface of the base substance to have a one-dimensional or two-dimensional ordered structure.
However, even by employing this method, it is difficult to obtain an electrode structure which is excellent in adhesiveness and allows a high capacitance to be obtained with a desired withstanding voltage being maintained.
Therefore, International Publication No. WO 2007/055121 (Patent Document 7) discloses a method in which in order to improve adhesiveness between an aluminum material as a base material and a dielectric layer, a dielectric precursor including valve metal is formed on the aluminum material, and thereafter, by heating the resultant in a hydrocarbon atmosphere, a dielectric layer including the valve metal is formed on a surface of the aluminum material and an interposing layer including aluminum and carbon is formed between the aluminum material and the dielectric layer, thereby enhancing the adhesiveness between the aluminum material and the dielectric layer.